1. Field of the Invention
The present invention relates to a multicrystalline silicone substrate and a process for roughening surface thereof, preferably used in a field of solar cell or the like.
2. Description of the Related Art
Solar cell is a device that converts light energy incident on its surface such as sunlight into electric energy. Various approaches have been attempted in order to improve efficiency in converting light energy into electric energy. One of such approaches is technique that reduces reflectance of light incident on the surface of the substrate. Reducing the reflectance of light incident on the surface allows the conversion efficiency into electric energy to be improved.
The main types of solar cells are classified by material used into crystalline silicon solar cells, amorphous silicon solar cells, compound based solar cells and the like. Most of the solar cells that are distributed in the market are crystalline silicon solar cells. The crystalline silicon solar cells are further classified into single crystal type and multicrystalline type. Single crystal silicon solar cells have the advantage that the conversion efficiency is relatively higher because of the high quality of the substrates. However, they have the disadvantage of high production cost of the substrates. In contrast, multicrystalline silicon solar cells have the disadvantage of inferior substrate quality making it difficult to improve conversion efficiency, while they have the advantage of low production cost. In addition, as a result of recent improvement in substrate quality of multicrystalline silicon solar cells and advancement of cell fabrication technology, conversion efficiency on the order of 18% has been achieved at laboratory level for multicrystalline silicon solar cells.
Meanwhile, multicrystalline silicon solar cells at mass-production level have been hitherto on the market because of their low cost. The demand has recently been further increasing under circumstances where environmental issues are of great concern. Accordingly, they are required to have higher conversion efficiency in addition to the low-cost benefit.
In fabricating a solar cell using a silicon substrate, etching a surface of the substrate in a predetermined condition with an alkaline aqueous solution such as a sodium hydroxide solution results in formation of textures on the surface, which reduces reflection of light at the surface to some extent.
When a single crystal silicon substrate with (100) orientation is used, a pyramidal pattern called textured structure can be formed uniformly on the surface of the substrate by such a process.
However, when fabricating a solar cell using a multicrystalline substrate, since texture etching with an alkaline aqueous solution depends on the crystal orientation, such a pyramidal pattern cannot be formed uniformly. For this reason, this poses the problem that the overall reflectance cannot be reduced effectively.
In order to overcome such a problem, preparing a surface having fine textures by means of Reactive Ion Etching has been proposed for the case where a solar cell is fabricated using a multicrystalline silicon substrate (for example, in the patent document [1]). By this method, fine textures can be uniformly formed independent of the irregular orientation of the crystals of multicrystalline silicon. In particular, reflectance can be more effectively reduced in solar cell using multicrystalline silicon.
[1] Japanese Patent Laid-Open Publication No. 1997-102625
Multicrystalline silicon substrates are generally fabricated by slicing a block or ingot of silicon obtained by a casting process. Widely used is slicing with use of an ID blade or multi-wire saw. Silicon substrates fabricated by such a process include a layer that is mechanically damaged by the slicing remaining on the surface, which causes degradation of the solar cell performance. The damaged layer needs to be removed in order to use the substrate for a solar cell. The thickness of the damaged layer is, although it varies depending on the machining process, generally on the order of 10 μm.
However, since the required depth to which the surface is etched by the foregoing reactive ion etching for forming fine textures is several microns at the largest, the damaged layer is hardly removed.
In order to form fine textures on the surface of a substrate for solar cell by reactive ion etching, the damaged layer is preferably removed prior to the formation.
Except for mechanical etching, various processes can be employed for the removal of the damaged layer. Although any of them may be used, in particular, wet etching with chemicals is a simple and easy process. In general, the easiest and inexpensive process is considered to be an etching with use of an alkaline aqueous solution including sodium hydroxide or potassium hydroxide.
However, it has been revealed that removal of the damaged layer by alkaline etching prior to reactive ion etching for forming fine textures sometimes results in formation of too complex irregularities on the substrate, and that the performance of solar cells fabricated using such substrates are degraded.
The object of the present invention is to provide a multicrystalline silicon substrate and a process for roughening a surface thereof that effectively improves the solar cell performance even in a condition of removal of the damaged layer by alkaline etching prior to reactive ion etching for forming fine textures.
A multicrystalline silicon substrate according to the present invention comprises: a substrate of multicrystalline silicon having relatively large irregularities formed on a surface thereof by etching with an alkaline aqueous solution; and a multiplicity of relatively fine textures formed over the relatively large irregularities by dry etching, wherein a ratio r expressed as r=a/b, which is the ratio between the length a of a virtual line connecting individual peaks of the relatively fine textures at a vertical cross section thereof and the length b of a straight line connecting the endpoints of the virtual line, is not smaller than 1 and smaller than 1.1.
This multicrystalline silicon substrate allows fine textures to be formed uniformly with even heights, thereby effectively reducing reflectance in a solar cell fabricated using the substrate. Solar cells with high conversion efficiency can thus be fabricated.
It is preferable that the fine textures have a height and a width of 2 μm or less, respectively. More preferably, the fine textures have a height and a width of 1 μm or less, respectively.
Preferably, the fine textures have a height-to-width aspect ratio of 2 or less. At aspect ratios greater than 2, the fine textures may suffer breakage during the production process causing large leak current to occur in the fabricated solar cell, which therefore fails to have good output performance.
A process for roughening a surface of a solar cell substrate according to the present invention comprises the steps of: etching a surface of a multicrystalline silicon substrate with an alkaline aqueous solution for forming relatively large irregularities having a surface area-to-planar surface area ratio R of larger than 1 and smaller than 1.1; and a dry etching step for forming a multiplicity of relatively fine textures over the relatively large irregularities.
This process allows fine textures to be formed uniformly, thereby effectively reducing reflectance in the solar cell fabricated using the substrate. Solar cells with high conversion efficiency can thus be fabricated.